The invention relates generally to heat transfer and refrigeration control systems. More particularly, the invention relates to control devices particularly suited for detecting characteristics, such as pressure, of the working fluid in such systems.
The basic building blocks of all refrigeration and heat transfer systems are well known and include a compressor, a condenser, an expansion means and an evaporator, all of which are connected in a fluid circuit having a working fluid such as halogen containing refrigerants such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and so forth. In an automotive air conditioning system, for example, the working fluid or refrigerant is typically in heat exchange with the vehicle compartment air by means of the evaporator. The liquid refrigerant turns to gas as it passes through the evaporator or endothermic heat exchanger thus absorbing heat from the ambient air. The gaseous refrigerant leaving the evaporator is drawn into the compressor through a suction line. The compressor pressurizes the gas which then passes through the condenser or exothermic heat exchanger where it is cooled back to a liquid state but is still under pressure. The pressurized liquid refrigerant is then passed through the expansion means, such as a valve, wherein the fluid pressure is adiabatically decreased prior to re-entering the evaporator.
Over the years, many different types of control mechanisms and monitoring devices have been used to regulate the operation of heat transfer or refrigeration systems. One of the more important functions required of a refrigeration control system is to monitor the high refrigerant pressure that exists between the compressor and the inlet port of the expansion means. This is important from a safety consideration to avoid excess pressure in the system. Being able to detect and control the working fluid pressure/temperature conditions is also useful for efficient operation of the refrigeration system by controlling subcooling of the pressurized working fluid. In the past, detecting pressure on the system high pressure side between the compressor and the expansion valve has been accomplished by such means as mechanical or electromechanical pressure transducers, pressure or temperature responsive valves, or simpler temperature sensors, the latter being used to approximate pressure based on the ambient temperature of the refrigerant. The mechanically responsive pressure sensors and valves tend to exhibit slow response times to working fluid conditions. More recently, the ready availability of electronic controllers such as microprocessors and other digital/analog controllers has provided the opportunity to electronically control and monitor the operation of the heat transfer system. This has an important benefit of being able to reduce the size and weight of the control system, and more importantly the cost, as well as improving the reliability and flexibility of the control functions.
Although the use of electronic controllers is well known, a suitable electronic pressure sensor has not yet been realized that is low cost but reliable and simple to incorporate into both new refrigeration systems as well as for retrofitting or upgrading older systems. Past efforts, for example, have attempted to use self-heated thermistors to boil the refrigerant and thus determine the saturation temperature or pressure based on the boiling point. This approach is inherently flawed, however, because a thermistor senses its own temperature, not the temperature of the refrigerant within which the thermistor is placed. By allowing the thermistor to self-heat by forcing constant current therethrough, the temperature measurement becomes inaccurate and unreliable. This occurs because the constant current may be greater or lesser than that required to cause boiling due to the variable degree of subcooling of the refrigerant. Not having enough power to cause boiling results in a low reading, whereas using more power than needed to cause boiling results in a high reading. These measurement errors are substantial in typical refrigeration and air conditioning systems because the amount of subcooling varies greatly with environmental conditions. The refrigerant boiling point technique is further flawed by the fact that the controller is only operated on the assumption that the thermistor is actually sensing the saturation temperature (i.e. the boiling point). The thermistor cannot detect the boiling event per se.
Accordingly, the need exists for a simple, reliable and accurate apparatus and method for detecting characteristics of a working fluid in a heat transfer system, particularly as those conditions relate to pressure in the high pressure regions of the fluid circuit.